The invention relates the field of optical feedthroughs. It relates more particularly to optical feedthroughs designed to interconnect two elements situated in respective environments that are sealed relative to each other.
Very often, apparatus containing optical fibers has the particularity that, for the needs of taking measurements, the fibers are situated in a wet or corrosive environment in which the pressure and the temperature can be high, while it is essential for the electronic equipment to which they are connected to be situated in a dry and clean environment and preferably under pressure conditions that are close to atmospheric conditions. Generally, it is necessary not only to solve problems of protecting the fibers, in order to guarantee effective optical transmission, but above all to solve problems of sealing the coupling between the fibers and the electronic equipment which is often placed in a protective enclosure. Thus, it is necessary to ensure that strict separation exists between the polluting environment surrounding the fibers and the environment inside the protective enclosure, while also allowing the fibers to penetrated into the protected environment.
Document U.S. Pat. No. 5,943,462 proposes an optical feedthrough that aims to solve the problem of sealing between the electronic equipment, situated in a protective housing, and the polluting environment surrounding the optical fiber. To that end, that document describes a feedthrough made up of two main portions. A first portion, through which a channel passes that serves to pass the fiber into a protective housing, is screwed via one end to said housing while the other end is screwed to the second portion, in the manner of a stopper. When the two portions are screwed together, they flatten the elastomer seals serving to provide sealing. The fiber also passes through said second portion.
That solution is not really satisfactory. That feedthrough is firstly particularly complicated and requires a large number of parts: it is therefore difficult and costly to manufacture. Secondly, when the fiber is situated in surroundings that are particularly polluting and/or that are under high pressure, the coupling provided by compressing the elastomer seals is not reliable. Such an optical feedthrough cannot withstand being used at particularly high pressures such as those encountered in hydrocarbon wells.
Other coupling solutions that are simpler have been developed. In particular, direct coupling is known that is constituted by brazing between the fiber and the protective housing: the fiber passes through the wall of the housing via an orifice that is filled in with brazing. Those solutions are indeed very simple and inexpensive to perform, but their weakness lies in the difficulty of assembling together effectively materials as different as the silica of the fiber and the metal of the brazing. In addition, those solutions raise obvious maintenance problems. Since a brazed coupling cannot be disassembled, if any one of the elements of the feedthrough is damaged, the entire feedthrough must be replaced, which is impractical and above all costly.
An object of the invention is to remedy those drawbacks by providing an optical feedthrough which guarantees excellent sealing between the electronic equipment and the environment surrounding the fiber under all conditions, the feedthrough being particularly well-suited to withstanding high stresses, in particular due to the temperature, to the pressure, or to the aggressiveness of said environment.
To this end, the invention provides an optical feedthrough including an optical fiber and a protective tube surrounding said fiber. In the invention, a seal extends inside the annular space between the protective tube and the optical fiber, the length of said seal being greater than 50 mm.
In this way, the feedthrough of the invention provides effective protection for the fiber against a polluting outside environment, while also distributing the stresses exerted by this environment, in particular those due to pressure. Since the seal extends inside the annular space between the tube and the fiber over quite a long distance, the chances of a leakage path being created by the seal being distributed poorly in the annular space are minimized, where such a leakage path would make it possible, in the event of the feedthrough breaking, for polluting fluids to penetrate through said annular space. The optical feedthrough of the invention is thus particularly well-suited to operating conditions under which the environment surrounding the electronic measurement instruments is very different from the environment in which the measurements are taken.
In a preferred embodiment of the invention, the seal extends inside the annular space between the protective tube and the optical fiber, over the entire length of said protective tube.
This embodiment makes it possible to guarantee total leaktightness for the connections made by the optical feedthrough of the invention. It is highly unlikely that polluting fluids can penetrate over such a length via the annular space between the tube and the fiber to reach the sensitive means to which the fiber is connected. The longer the seal, the lower the chances of a path being created to said means by said seal being poorly distributed in the annular space. In addition, the presence of the seal over the entire length of the fiber makes it possible to impart greater strength to the feedthrough.
In an advantageous embodiment of the invention, the protective tube is made of metal.
This embodiment makes it possible firstly to stiffen the optical feedthrough as a whole, and thus to increase its resistance to shocks during handling. In addition, when the optical feedthroughs of the invention are used in wells containing hydrocarbons or water, the protective tube is particularly effective in these highly abrasive and corrosive environments for avoiding any breaking of the optical fiber. Furthermore, since the protective housings surrounding the electronic means connected to the fiber are very often made of metal, it is easy and inexpensive to provide reliable coupling between the tube and the housings if they are made of the same material.
In a preferred embodiment of the invention, the diameter of the fiber is close to the inside diameter of the protective tube, so that the thickness of the seal is preferably less than 0.05 mm.
It is preferable for the thickness of the annular space to be limited so as to guarantee that the fiber is placed ideally inside the protective tube. To reduce the probability of having a seal-free path through the annular space, it is necessary to ensure that the fiber occupies as much space as possible inside the tube, and thus for the seal also to be distributed uniformly over the small available space, without an undesirable build-up in one place to the detriment of some other place, because of the space left unoccupied by the fiber.
In a preferred embodiment of the invention, the seal is a seal of thermo-settable adhesive, said seal being polymerized in the protective tube at a first temperature so that, at a second temperature that is lower than said first temperature, said seal is compressed between the walls of said tube and the optical fiber.
In this way, since the seal is polymerized inside the annular space at the maximum temperature at which the feedthrough is to be used, then, at the time it is polymerized, said seal occupies all of the available space in said annular space with the expansion of the protective tube being at its maximum. In which case, the seal is compressed against the walls of the tube while the feedthrough is being used over an entire range of temperatures extending to the maximum temperature (the polymerization temperature). It is thus possible to limit the risks of the seal breaking, and particularly the risks of cracks forming in it, while maximizing the sealing of the annular space when the tube and the fiber are in the aggressive environment.
This embodiment also makes it possible to accommodate as well as possible the differences in materials, and thus the differences in thermal expansion between the optical fiber and the protective tube. The optical fiber is made of silica whereas the tube is made of a completely different material, e.g. a metal. In which case, when the environment surrounding the feedthrough is at high temperatures or at high pressures, the tube tends to expand to a greater extent than the extent to which the fiber expands. If the seal were not under compression, but rather at its maximum expansion, it would not be able to withstand such differential expansion, and it would break. Whereas, in this embodiment, the seal expands naturally inside the annular space so that, without breaking, it compensates for the differential expansion between the tube and the fiber. The leaktightness of the annular space is thus guaranteed even when the feedthrough is highly stressed, in particular due to temperature or pressure.
In another preferred embodiment of the invention, the seal extends inside the annular space between the optical fiber and the protective tube, at least from a first end of said fiber to a preferred zone of weakness of said protective tube.
This solution makes it possible to control the place at which any breaking takes place, and consequently to provide enough material to guarantee sealing when the fiber is connected to electronic measurement means. In which case, it is necessary merely for the seal to extend at least as far as said preferred zone of weakness.
In an advantageous embodiment of the invention, the second end of the fiber is connected to an element situated outside a housing, said feedthrough including closure means co-operating with the protective tube so as to seal off the inside of said housing relative to the outside environment. In this feedthrough, the first end of the fiber is situated inside the housing, and the preferred zone of weakness is situated outside said housing.
The characteristics of the optical feedthrough of the invention make it possible to provide effective sealing between an environment situated inside a protective housing, containing electronic means for example, and the outside of the housing, where conditions are such that they would damage said electronic means. By placing the preferred zone of weakness outside the housing, and by ensuring that the seal extends from the inside of the housing to said zone of weakness, the chances of elements penetrating into the housing in the event that the feedthrough breaks are lowered considerably.
In an embodiment of the invention, the closure means include a sleeve through which the protective tube passes.
This embodiment enables sealing relative to the outside environment to be provided easily and reliably where the protective tube containing the fiber passes into the housing. The sleeve makes it possible to increase the stiffness of the assembly in the vicinity of this critical zone. In addition, the use of the sleeve makes it possible to disassemble the optical feedthrough of the invention. This makes it easy to change the feedthrough in the event that it is damaged, and thus facilitates maintenance of the tool that it equips.
In a preferred embodiment of the invention, a first end of the sleeve is situated outside the housing and is coupled to the protective tube so that the coupling corresponds to the preferred zone of weakness of said tube.
This configuration makes it possible to control the place at which any breaking of the optical feedthrough takes place, and consequently to control the constraints for sealing the feedthrough. In addition, since the zone of weakness is offset relative to the sealing zone (the other end of the sleeve), a xe2x80x9csafetyxe2x80x9d length is retained that corresponds to the length of the feedthrough, in order to preserve the leaktightness of the zone in which the protective tube and the fiber penetrate into the housing.
In another advantageous embodiment of the invention, the sleeve is made of metal, and it is connected by brazing to the protective tube which is also made of metal.
In this way, the sleeve imparts additional stiffness to the coupling to the housing. Since the housing is very often made of metal, it is easy to couple together the elements, the solution of brazing being particularly inexpensive.
Other advantages and characteristics of the invention appear more clearly from the following description given with reference to the sole accompanying drawing which is a longitudinal section view of an embodiment of an optical feedthrough of the invention.